Fuel assembly skeleton with structural and non-structural top nozzle/guide thimble joints

ABSTRACT

A fuel assembly skeleton includes a top nozzle, a number of longitudinally-extending guide thimbles having upper ends, a number of joints interconnecting the top nozzle and the upper ends of the guide thimbles. All but a few of the joints are structural joints providing rigid connections between all but a corresponding few of the guide thimble upper ends and the top nozzle. Each of the other few joints is a non-structural joint providing a non-rigid connection between a guide thimble upper end and the top nozzle. The number of structural joints constitutes at least a substantial majority of the joints and, in any event, the number is at least three times the number of non-structural joints. Each non-structural joint is composed of a hollow sleeve rigidly connected to the top nozzle and disposed in a slip fit relationship about the guide thimble upper end. The sleeve is bulge fitted in a passageway defined in the top nozzle.

CROSS REFERENCE TO RELATED APPLICATIONS

Reference is hereby made to the following copending applications dealingwith related subject matter and assigned to the assignee of the presentinvention:

1. "Nuclear Reactor Fuel Assembly With A Removable Top Nozzle" by JohnM. Shallenberger et al, assigned U.S. Ser. No. 644,758 and filed Aug.27, 1984, now U.S. Pat. No. 4,631,168.

2. "Locking Tube Removal And Replacement Tool And Method In AReconstitutable Fuel Assembly" by John M. Shallenberger et al, assignedU.S. Ser. No. 670,418 and filed Nov. 9, 1984, now U.S. Pat. No.4,639,998.

3. "Top Nozzle Removal And Replacement Fixture And Method In AReconstitutable Fuel Assembly" by John M. Shallenberger et al, assignedU.S. Ser. No. 670,729 and filed Nov. 13, 1984.

4. "Locking Tube Insertion Fixture And Method In A Reconstitutable FuelAssembly" by John M. Shallenberger et al, assigned U.S. Ser. No. 689,696and filed Jan. 8, 1985, now U.S. Pat. No. 4,638,556

5. "Locking Tube Removal Fixture And Method In A Reconstitutable FuelAssembly" by John M. Shallenberger et al, assigned U.S. Ser. No. 694,762and filed Jan. 28, 1985.

6. "Reusable Locking Tube In A Reconstitutable Fuel Assembly" by John M.Shallenberger et al, assigned U.S. Ser. No. 719,108 and filed Apr. 2,1985.

7. "Fixture And Method For Rectifying Damaged Guide Thimble InsertSleeves In A Reconstitutable Fuel Assembly" by John M. Shallenberger etal, assigned U.S. Ser. No. 797,337 and filed Nov. 13, 1985.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to fuel assemblies for nuclearreactors and, more particularly, is concerned with a reconstitutablefuel assembly skeleton which after being repaired has both structuraland non-structural top nozzle/guide thimble joints.

2. Description of the Prior Art

In most nuclear reactors, the reactor core is comprised of a largenumber of elongated fuel assemblies. Conventional designs of these fuelassemblies include a plurality of fuel rods and control rod guidethimbles held in an organized array by grids spaced along the fuelassembly length and attached to the control rod guide thimbles. Top andbottom nozzles on opposite ends of the fuel assembly are secured to theguide thimbles which extend slightly above and below the ends of thefuel rods. At the top end of the fuel assembly, the guide thimbles areattached in passageways provided in the adapter plate of the top nozzle.The guide thimbles may each include an upper insert or sleeve forattachment to the top nozzle.

During operation of such fuel assembly in a nuclear reactor, a few ofthe fuel rods may occasionally develop cracks along their lengthsresulting primarily from internal stresses, thus establishing thepossibility that fission products having radioactive characteristics mayseep or otherwise pass into the primary coolant of the reactor. Inresponse to the high costs associated with replacing fuel assembliescontaining defective fuel rods, recently, reconstitutable fuelassemblies have been developed. The general approach to making a fuelassembly reconstitutable is to provide it with a removable top nozzle.

One approach to top nozzle removal is illustrated and disclosed inEuropean Patent Application No. 0 036 821. In one embodiment of thisapproach, as shown in FIGS. 3, 6 and 7 of the application, the guidethimbles are severed just below the top nozzle adapter plate to allowremoval of the top nozzle. Then, a new top nozzle equipped with shorttubes having diameters slightly larger than the outside diameter of theguide thimbles are applied to the upper severed ends of the guidethimbles and bulged fitted thereto. However, this approachdisadvantageously requires the use of a new top nozzle and only allowsthe fuel assembly to be reconstituted once.

Another approach to reconstitutable fuel assembly construction isillustrated and described in the first application cross-referencedabove. It incorporates a releasable attaching structure for removablymounting the top nozzle on the upper ends of the control rod guidethimbles. Various tools and fixtures used in releasing and installingthe attaching structure for removing and replacing the top nozzle areillustrated and described in the second through fifth cross-referencedapplications. A recently devised, reusable locking tube for theattaching structure is illustrated and described in the sixthapplication cross-referenced above.

The attaching structure includes a plurality of outer sockets defined inan adapter plate of the top nozzle, a plurality of inner sockets witheach formed on the upper end of one of the guide thimbles, and aplurality of removable locking tubes inserted in the inner sockets tomaintain them in locking engagement with the outer sockets. Each outersocket is in the form of a passageway through the adapter plate whichhas an annular groove. Each inner socket is in the form of a hollowupper end portion of the guide thimble having an annular bulge whichseats in the annular groove when the guide thimble end portion isinserted in the adapter plate passageway. A plurality of elongated axialslots are provided in the guide thimble upper end portion to permitinward elastic collapse of the slotted portion so as to allow the largerbulge diameter to be inserted within and removed from the annularcircumferential groove in the passageway of the adapter plate. In suchmanner, the inner socket of the guide thimble is inserted into andwithdrawn from locking engagement with the outer socket.

The locking tube is inserted from above the top nozzle into a lockingposition in the hollow upper end portion of the guide thimble formingthe inner socket. When inserted in its locking position, the lockingtube retains the bulge of the inner socket in its expanded lockingengagement with the annular groove and prevents the inner socket frombeing moved to a compressed releasing position in which it could bewithdrawn from the outer socket. In such manner, the locking tubesmaintain the inner sockets in locking engagement with the outer sockets,and thereby the attachment of the top nozzle on the upper ends of theguide thimbles.

Although unlikely, the possibility exists that during a fuel assemblyreconstitution operation, while the top nozzle is removed from theirradiated fuel assembly to provide access to any defective fuel rod,the upper end portion, or insert sleeve, of the guide thimble could beinadvertently damaged by tooling or by a dropped object which strikesthe guide thimble upper portion. Should such a mishap occur, the fuelassembly could be rendered unacceptable for further reactor service ifthe damaged portion cannot enter the respective top nozzle adapter platepassageway and be properly secured by the respective locking tube.

Consequently, there is a need for a workable repair approach to rectify(i.e., repair or remove) and accommodate damaged guide thimble upper endportions which effectively addresses potential customer concerns relatedto improbable, but possible, mishaps which could jeopardize successfulfuel reconstitution.

SUMMARY OF THE INVENTION

The present invention together with the invention of the seventhapplication cross-referenced above are designed to satisfy theaforementioned needs by providing contingency reconstitution equipmentand a top nozzle/guide thimble joint modification designed to rectify(i.e., repair or remove) and accommodate damaged guide thimble upper endportions.

The invention of the last cross-referenced application relates to afixture and method for rectifying damaged guide thimble upper endportions. In particular, a remotely-operated fixture is operable torepair moderately damaged guide thimble portions and cut off irreparablydamaged portions, thus permitting the remounting of the top nozzle onthe repaired and severed portions of the guide thimbles and thecontinued use of the reconstituted fuel assembly in the reactor.

In cases where the damaged guide thimble portions are irreparable andmust be cut off, the present invention presents a modified top nozzleattachment concept compatible with fuel assembly skeletons containingseveral severed guide thimble upper end portions, or insert sleeves.Specifically, in the modified top nozzle, a non-structural joint issubstituted for the severed guide thimble portion which previouslyprovided a structural joint with the top nozzle adapter plate. Dependingupon the particular fuel assembly configuration, at least four to sixremovable top nozzle/guide thimble structural joints can be eleminatedwithout increasing the load carried by the remaining structural jointsto an unacceptable level.

Accordingly, the present invention sets forth in a fuel assemblyskeleton including a top nozzle and a plurality oflongitudinally-extending guide thimbles having upper ends, meansinterconnecting the top nozzle and upper ends of the guide thimbleswhich includes: (a) a plurality of structural joints; and (b) at leastone non-structural joint. The non-structural joint is composed of amember, such as a hollow sleeve, rigidly connected to one of the topnozzle and the upper end of one of the guide thimbles and is disposed ina slip fit relationship to the other of the top nozzle and the guidethimble upper end. The sleeve has an upper portion joined to the topnozzle and of a diameter size substantially the same as that of theguide thimble upper end, and a lower portion of a diameter size slightlylarger than that of the guide thimble upper end for slidably receivingthe guide thimble upper end therein in a close fitting relationship.

The present invention also relates to a fuel assembly skeleton, whichcomprises the combination of: (a) a top nozzle; (b) a plurality oflongitudinally-extending guide thimbles having upper ends; and (c) aplurality of joints interconnecting the top nozzle and the upper ends ofthe guide thimbles. All but at least one of the joint are structuraljoints providing rigid connections between all but at least one of theguide thimble upper ends and the top nozzle. The one joint is anon-structural joint providing a non-rigid connection between the oneguide thimble upper end and the top nozzle. More particularly, thenumber of structural joints constitutes at least a substantial majorityof the joints and, in any event, is at least three times the number ofthe non-structural joints.

These and other advantages and attainments of the present invention willbecome apparent to those skilled in the art upon a reading of thefollowing detailed description when taken in conjuction with thedrawings wherein there is shown and described an illustrative embodimentof the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the course of the following detailed description, reference will bemade to the attached drawings in which:

FIG. 1 is a side elevational view, with parts partially sectioned andbroken away for purposes of clarity, of a reconstitutable fuel assemblyin which an irreparably damaged top nozzle/guide thimble structuraljoint of the assembly skeleton can be rectified by replacement with anon-structural joint in accordance with the principles of the presentinvention.

FIG. 2 is a sectioned, exploded view of the components of the attachingstructure associated with the top nozzle and the guide thimbles of thereconstitutable fuel assembly.

FIG. 3 is a top plan view, as seen along line 3--3 of FIG. 2, lookingdown on the top of the upper end of the control rod guide thimble.

FIG. 4 is a side elevational view of the reusable locking tube of theattaching structure by itself.

FIG. 5 is a top plan view, as seen along line 5--5 of FIG. 4, lookingdown on the top of the reusable locking tube.

FIG. 6 is an enlarged fragmentary view of the upper end of the improvedreusable locking tube of FIG. 2, showing a slightly outwardly tapered orflared upper peripheral edge portion thereof.

FIG. 7 is a sectional view of the components of the attaching structureof FIG. 2 being assembled together.

FIG. 8 is a fragmentary side elevational view of a reconstitutable fuelassembly disposed in a submerged work station, with the damagerectifying fixture of the invention of the last cross-referencedapplication positioned above the work station and fuel assembly inalignment therewith, and with the top nozzle of the fuel assembly beingremoved to expose guide thimbles having the different possible types ofupper end portion damage which can be rectified by operation of thefixture.

FIG. 9 is an enlarged top plan view of the base of the fixture of FIG.8, illustrating the base with the various repair and cutting tools andpositioning elements of the fixture removed from the base.

FIG. 10 is a sectional view of the fixture base as taken along line10--10 of FIG. 9.

FIG. 11 is an enlarged elevational view, with portions broken away andsectioned, of the positioning element in the form of a non-compressersocket which was shown mounted to the fixture base in FIG. 8 but removedfrom the fixture base in FIG. 9.

FIG. 12 is a cross-sectional view of the positioning element as takenalong line 12--12 of FIG. 11.

FIG. 13 is an enlarged elevational view, with portions broken away andsectioned, of the compresser socket tool which was shown mounted to thefixture base in FIG. 8 but removed from the fixture base in FIG. 10.

FIG. 14 is a cross-sectional view of the compresser socket tool as takenalong line 14--14 of FIG. 13.

FIG. 15 is an enlarged elevational view of the expander pin tool whichwas shown mounted to the fixture base in FIG. 8 but removed from thefixture base in FIG. 10.

FIG. 16 is a cross-sectional view of the expander pin tool as takenalong line 16--16 of FIG. 15.

FIG. 17 is a top plan view of the damage rectifying fixture mounted atthe submerged work station in operative starting position overlying afuel assembly whose damaged guide thimble upper end portions or insertsleeves are to be repaired or removed depending on the type of damageincurred by the sleeves.

FIG. 18 is a sectional view of the submerged work station and the damagerectifying fixture in operative starting position over the fuel assemblyas taken along line 18--18 of FIG. 17.

FIGS. 19 through 21 are sectional views similar to that of FIG. 18, butshowing sequential stages in the rectification of damaged guide thimbleinsert sleeves by the fixture, including in FIG. 19, the lowering of thefixture to repair inserts with respective outwardly and inwardly flaredor bent upper end segments and to sever an insert with irreparabledamage; in FIG. 20, the removal of the fixture with the severed insertportion being carried therewith; and in FIG. 21, the replacement of thetop nozzle back on the guide thimbles with the top nozzle carrying anadapter sleeve which will form a modified non-structural joint with thesevered guide thimble insert sleeve in accordance with the presentinvention.

FIG. 22 is an enlarged elevational view, with portions broken away andsectioned, of the adapter sleeve which is used to form the modifiednon-structural joint in accordance with the present invention.

FIG. 23 is an enlarged fragmentary view, in section, of the top nozzleshown in FIG. 21, illustrating the bulge fitted connection of theadapter sleeve in the passageway groove of the top nozzle adapter plate.

FIG. 24 is a view similar to that of FIG. 23, but showing the adaptersleeve inserted over the severed guide thimble insert sleeve and thelocking tube installed in the adapter sleeve.

DETAILED DESCRIPTION OF THE INVENTION

In the following description, like reference characters designate likeor corresponding parts throughout the several views. Also in thefollowing description, it is to be understood that such terms as"forward", "rearward", "left", "right", "upwardly", "downwardly", andthe like, are words of convenience and are not to be construed aslimiting terms.

IN GENERAL

Referring now to the drawings, and particularly to FIG. 1, there isshown an elevational view of a reconstitutable nuclear reactor fuelassembly, represented in vertically foreshortened form and beinggenerally designated by the numeral 10. Basically, the fuel assembly 10includes a lower end structure or bottom nozzle 12 for supporting theassembly on the lower core plate (not shown) in the core region of areactor (not shown), and a number of longitudinally extending guidetubes or thimbles 14 which project upwardly from the bottom nozzle 12.The assembly 10 further includes a plurality of transverse grids 16axially spaced along the guide thimbles 14 and an organized array ofelongated fuel rods 18 transversely spaced and supported by the grids16. Also, the assembly 10 has an instrumentation tube 20 located in thecenter thereof and an upper end structure or top nozzle 22 removablyattached to the upper ends of the guide thimbles 14, in a manner fullydescribed below, to form an integral assembly capable of beingconventionally handled without damaging the assembly parts.

As mentioned above, the fuel rods 18 in the array thereof in theassembly 10 are held in spaced relationship with one another by thegrids 16 spaced along the fuel assembly length. Each fuel rod 18includes nuclear fuel pellets 24 and the opposite ends of the rod areclosed by upper and lower end plugs 26,28 to hermetically seal the rod.Commonly, a plenum spring 30 is disposed between the upper end plug 26and the pellets 24 to maintain the pellets in a tight, stackedrelationship within the rod 18. The fuel pellets 24 composed of fissilematerial are responsible for creating the reactive power of the nuclearreactor. A liquid moderator/coolant such as water, or water containingboron, is pumped upwardly through the fuel assemblies of the core inorder to extract heat generated therein for the production of usefulwork.

To control the fission process, a number of control rods 32 arereciprocally movable in the guide thimbles 14 located at predeterminedpositions in the fuel assembly 10. Specifically, the top nozzle 22includes a rod cluster control mechanism 34 having an internallythreaded cylindrical member 36 with a plurality of radially extendingflukes or arms 38. Each arm 38 is interconnected to a control rod 32such that the control mechanism 34 is operable to move the control rods32 vertically in the guide thimbles 14 to thereby control the fissionprocess in the fuel assembly 10, all in a well-known manner.

TOP NOZZLE ATTACHING STRUCTURE

As illustrated in FIGS. 2 and 7, the top nozzle 22 has a lower adapterplate 40 with a plurality of control rod passageways 42 (only one beingshown) formed through the adapter plate. The control rod guide thimbles14 have their uppermost end portions 44 coaxially positioned within thepassageways 42 in the adapter plate 40. For gaining access to the fuelrods 18 in reconstitution of the fuel assembly 10, the adapter plate 40of the top nozzle 22 is removably connected to the upper end portions 44of the guide thimbles 14 by an attaching structure, generally designated46, which provides a plurality of structural joints between the topnozzle and the guide thimbles of the fuel assembly skeleton. Theattaching structure 46 is generally the same as illustrated anddescribed in the sixth application cross-referenced above, but will bedescribed herein to the extent necessary to facilitate an understandingof the present invention.

As best seen in FIGS. 2 through 7, the top nozzle attaching structure 46of the reconstitutable fuel assembly 10 includes a plurality of outersockets (only one being shown) defined in the top nozzle adapter plate40 by the plurality of passageways 42 (also only one being shown) whicheach contains an annular circumferential groove 48 (only one beingshown), a plurality of inner sockets (only one being shown) defined onthe upper end portions 44 (only one being shown) of the guide thimbles14, and a plurality of removable and reusable locking tubes 50 (only onebeing shown) inserted in the inner sockets to maintain them in lockingengagement within the outer sockets.

Each inner socket is defined by an annular circumferential bulge 52 onthe hollow upper end portion 44 of one guide thimble 14 only a shortdistance below its upper edge 54. A plurality of elongated axial slots56 are formed in the upper end portion 44 of each guide thimble 14 topermit inward elastic collapse of the slotted end portion to acompressed position so as to allow the circumferential bulge 52 thereonto be inserted within and removed from the annular groove 48 via theadapter plate passageway 42. The annular bulge 52 seats in the annulargroove 48 when the guide thimble end portion 44 is inserted in theadapter plate passageway 42 and has assumed an expanded position. Insuch manner, the inner socket of each guide thimble 14 is inserted intoand withdrawn from locking engagement with one of the outer socket ofthe adapter plate 40.

More particularly, the axially extending passageway 42 in the adapterplate 40 which defines the outer socket is composed of an upper bore 58and a lower bore 60. The lower bore 60 is of considerably greater axiallength than the upper bore 58 and contains the annular groove 48 whichis spaced a short distance below a ledge 62 formed at the intersectionof the upper and lower bore 58,60. The lower bore 60 has a diameterwhich is greater than that of the upper bore 58; therefore, the ledge 62faces in a downward direction. The primary purpose of the ledge 62 is toserve as a stop or an alignment guide for proper axial positioning ofthe upper end portion 44 in the passageway 42 when the inner socket isinserted into the outer socket. As seen in FIG. 7, the upper edge 54 ofthe guide thimble 14 abuts the ledge 62.

Finally each locking tube 50 is inserted from above the top nozzle 22into its respective locking position in the hollow upper end portion 44of one guide thimble 14 forming one inner socket. When the locking tube50 is inserted in its locking position, as seen in FIG. 7, it retainsthe bulge 52 of the inner socket in the latter's expanded lockingengagement with the annular groove 48 and prevents the inner socket frombeing moved to its compressed releasing position in which it could bewithdrawn from the outer socket. In such manner, each locking tube 50maintains its respective one inner socket in locking engagement with theouter socket, and thereby retain the structural joint formed by theattachment of the adapter plate 40 of the top nozzle 22 on the upper endportion 44 of each guide thimble 14 in an assembled rigid form.

Also, the locking tube 50 includes a slightly outwardly flared (forinstance 1-2 degrees) upper peripheral marginal edge portion 64 (FIG.6), which has an outer diameter slightly larger than the diameter of theupper bore 58, and a pair of small dimples 66 (being shown exaggeratedin size) to secure it in its locking position. The flared edge portion64 provides a tight frictional fit with the adapter plate 40, whereasthe dimples 66 extend into the circumferential groove 48 defined in theadapter plate passageway 42. The locking tube 50 will yield as a wholein allowing withdrawal of the dimples 66 from the groove 48 and the tubefrom its locking position, after which the locking tube 50 will springback to its original shape.

APPARATUS FOR RECTIFYING DAMAGED GUIDE THIMBLE UPPER ENDS

Turning now to FIG. 8, for effectuating inspection, removal, replacementand/or rearrangement of fuel rods 18 contained in the reconstitutablefuel assembly 10, the irradiated assembly must be removed from thereactor core and lowered into a work station 68 by means of a standardfuel assembly handling tool (not shown). In the work station 68, thefuel assembly 10 is submerged in coolant and thus maintenance operationsare performed by manipulation of remotely-controlled submersibleequipment. One component of such equipment is used for removing andreplacing the locking tubes and another component is used, after thelocking tubes have been removed, for removing and subsequently replacingthe top nozzle 22 from and on the guide thimbles 14 of thereconstitutable fuel assembly 10. These components can take the form ofthe ones described and illustrated in the second through fifthapplication cross-referenced above.

The work station 68 includes a pair of elongated bullet-nose guidemembers 70 which are mounted on, and project upwardly from, a pair ofdiagonal corners of a top flange 72 of the work station 52. The guidemembers 70 assist in alignment of the various components used to removethe locking tubes 50 and remove and replace the top nozzle 22. Also,included in the work station 68 are opposed pairs of movable pads 74(only one pair being shown) that are mounted on and project through theside walls 76 of the work station at the elevation of the uppermost grid16 of the fuel assembly 10. The pads 74 are advanced inwardly bycylinders 78, also mounted on the side walls 76, to bear against eachside of the grid 16 and take up the clearance between the station 68 andthe assembly 10, and thus maintain it in a fixed relation to the workstation.

In FIG. 8, there is also depicted three general types of damage which,although improbable, could possibly happen to the upper end portions 44(or insert sleeves) of the guide thimbles 14, and particularly to thesegments 80 thereof being separated by axial slots 56, should they behit by a dropped object or bumped by a piece of equipment duringreconstitution of the fuel assembly 10 when the top nozzle 22 is removedfrom the assembly and the guide thimbles 14 are exposed from above. Thefirst type of segment damage, being represented on guide thimble upperend portion 44a, involves one (or more) of the segments 80a being flaredor bent outwardly away from its desired vertical position. In suchposition of the segment 80a, the damaged upper end portion 44a of theguide thimble 14a will not fit into the corresponding passageway 42 inthe top nozzle adapter plate 40 when the top nozzle is replaced on theguide thimbles. The second type of segment damage, being represented onguide thimble upper end portion 44b, involves one (or more) of thesegments 80b being bent inwardly away from its desired verticalposition. In such position of the segment 80b, the damaged upper endportion 44b of the guide thimble 14b may fit into the correspondingadapter plate passageway 42, but will not seat properly in the groove 48therein nor will it receive one of the locking tubes 50.

However, in view of the invention of the last cross-referencedapplication, each of the first and second types of segment damage areconsidered to be directly repairable, as will be explained shortly. Suchis not the case with the third type of damage, being represented on theguide thimble upper end portion 44c. It involves one (or more) of thesegments 80c being too severly bent outwardly or damaged to be directlyrepairable, such as by bending it back to its desired vertical position.Nonetheless, the cross-referenced invention makes provision forrectifying the damage by removing the affected section of the upper endportion 44c, as also will be explained shortly. Then, in accordance withthe present invention, which will also be described below, a substituteor replacement structure is provided for the removed section.

Turning now to FIGS. 9 through 16, as well as to FIG. 8, there is showna fixture, constituting the preferred embodiment of the cross-referencedinvention and being generally designated 82, which can be used duringreconstitution of the fuel assembly 10, if the need arises, to rectify(i.e., repair or remove) damage to any of the axially segmented upperend portions 44 of the fuel assembly guide thimbles 14. With the topnozzle 22 removed from the fuel assembly 10, it is possible althoughhighly improbable that, during fuel assembly reconstitution operations,at least one of the guide thimble segments 80 may incur one of the threedifferent types of repairable and irreparable damage which were brieflydescribed above.

As shown particularly in FIGS. 9 and 10, the damage rectifying fixture82 includes a base 84 made up of a generally square plate 86 enclosed byand connected with a frame 88. The plate 86 has a plurality of toolpositioning openings 90 defined therein in a pattern matched with thatof the guide thimbles 14. The frame 88 has means for mounting the base84 on the work station 68 in the form of a pair of aligning holes 92defined in two opposite corners of the frame for receiving the guidemembers 70 which project upward from the top flange 72 of the workstation. When the base 84 is positioned above the station 68, as seen inFIG. 8, and then lowered to just receive the guide members 70 of thestation upwardly through its aligning holes 92, as seen in FIG. 18, thetool positioning openings 90 of the base 84 are then disposed inalignment with the exposed upper end portions 44 of the fuel assemblyguide thimbles 14.

Further, the damage rectifying fixture 82 incorporates a set of threedifferent tools 94,96,98 corresponding to the three different types ofdamage which can be incurred by the guide thimbles 14, as describedabove. Each of the tools 94,96,98 is selectively mountable to the baseplate 86 at any one of its openings 90 by a threaded nut and boltconnection 100 provided on the upper end of the tool. Basically,therefore, each tool is adapted to operate to rectify a different one ofthe types of repairable and irreparable damage to the segments 80 on theupper end portions 44 of the guide thimbles 14. (It will be recalledthat the portions 44, which commonly take the form of insert sleeves,form the structural joints with the top nozzle adapter plate 40, whenthe top nozzle 22 is remounted on the guide thimbles 14, as wasdescribed earlier).

In addition to the damage rectifying tools 94,96,98, the fixture 82incorporates a group of positioning elements 102 mounted in selectedones of the base openings 90 which will not be occupied by any of thetools. Each of positioning elements 102, as best seen in FIGS. 11 and12, is a non-compresser socket having a cylindrical body 104 attached tothe base plate 86 at one of the openings 90 by a threaded connection 106and a cylindrical cavity 108 defined in the lower end of the body. Thebody 104 has outwardly tapered lower guide opening 109 defined thereinwhich leads into the cavity 108. The cavity 108 is of a diameter sizeslightly larger than the outside diameter size of an undamaged one ofthe upper end portions 44 which adapts the positioning element 102 tomount upon the undamaged guide thimble upper end portion 44d, as seen inFIG. 19. Thus, selection of the specific positioning holes 90 in thefixture base 84 for attachment of the non-compresser socket elements 102shall correspond to undamaged guide thimble portions 44d, preferablyfour in number (only one is shown in the drawings for purposes ofclarity), with one in each quadrant of the fuel assembly skeleton. Thefunction of the positioning elements 102 is to precisely locate thefixture 82 relative to the guide thimble upper end portions 44 forensuring accurate performance of the damage rectifying operationscarried out by the tools 94,96,98. Specifically, the purpose of thesocket elements 102 is to locate the fixture 82 at a precise distanceabove and parallel to the normal plane of the top edges 54 of the guidethimbles.

In FIGS. 13 and 14, as well as in FIG. 8, the damage rectifying tool 94is the one designed to repair the first type of damage described abovewith respect to one (or more) of the segments 80a on the guide thimbleupper end portion 44a. The tool 94 takes the form of a compressor sockethaving a cylindrical body 110 attached to the base plate 86 at one ofthe openings 90 by the threaded connection 100 and a cylindrical cavity112 defined in the lower end of the body. The body 110 has an outwardlytapered lower edge 114 defining a guide opening 114 leading into thecavity 112. The cavity 112 is of a diameter size generally slightly lessthan the outside diameter size of an undamaged one of the upper endportions 44 which adapts the compresser socket tool 94 to operate torectify repairable damage to the guide thimble upper end portion 44ahaving an outwardly bent segment 80a by compressing the segment 80ainward to its original position, as seen in FIG. 19. Thus, compressersocket tools 94 would be attached at positioning holes 90 of the base 84corresponding to those of the guide thimbles having damage of the firsttype.

In FIGS. 15 and 16, as well as in FIG. 8, the damage rectifying tool 96is the one designed to repair the second type of damage described abovewith respect to one (or more) of the segments 80b on the guide thimbleupper end portion 44b. The tool 96 takes the form of an expander pinhaving a cylindrical body 118 attached to the base plate 86 at one ofthe openings 90 by the threaded connection 100 and a reduced diameterlower section 120 with a tapered guide end 122 defined on the lower endof the body. The body 118 has a diameter size generally slightly lessthan the inside diameter of an undamaged one of the upper end portions44 which adapts the expander pin tool 96 to operate to rectifyrepairable damage to the guide thimble upper end portion 44b having aninwardly bent segment 80b by expanding the segment 80b outward to itsoriginal position, as seen in FIG. 19. Thus, expander pin tools 96 wouldbe attached at positioning holes 90 of the base 84 corresponding tothose of the guide thimbles having damage of the second type.

Finally, the tool 98, being the one to rectify the third type of damage,preferably takes the form of a precision internal cutter, such as eitherone of the two embodiments disclosed and illustrated in U.S. applicationSer. No. 649,864 and assigned to the assignee of the present invention,the disclosure of which is incorporated by reference. The cutter tool 98is operable to rectify irreparable damage to one (or more) of the bentsegments 80c of the guide thimble upper end portion 44c by severing andcapturing a predetermined section 124 of the guide thimble upper endportion 44c which includes the irreparably damaged segment 80c, as seenin FIGS. 19 and 20.

In FIGS. 17 and 18, the damage rectifying fixture 82 is shown positionedin the work station 68 with the top ends of the guide members 70extending through the aligning holes 92 of the fixture base 84. Along-handled lifting tool (not shown) threaded at its lower end 126 intoa threaded central bore 128 in the base plate 86 is used to more thefixture 82 into the starting position shown in FIGS. 17 and 18, and alsoto more the fixture further toward the guide thimbles 14 to carry outthe rectifying operations.

Normally, only one type of damage condition would exist at one time, andeven then infrequently, in a given fuel assembly and thus each damagerectifying operation using an appropriate one of the tools 94,96,98would be carried out separately. However, for purposes of explaining thethree types of damage and the three tools 94,96,98 which can be used bythe fixture 82 of the present invention to rectify the damage, the threedifferent damage conditions are shown as existing at the same time onthe fuel assembly guide thimbles 14 in FIG. 18 and will thus berectified generally at the same time by the three different tools.Therefore, in FIGS. 17 and 18, the fixture 82 is illustrated with onecompresser socket tool 94, one expander pin tool 96, one cutter tool 98and four non-compresser socket elements 102 aligned with correspondingdamaged and undamaged guide thimble upper end portions 44a-c and 44d.Note that the four socket elements 102 are mounted in four differentquadrants of the base plate 86 corresponding to the four differentquadrants of the fuel assembly 10.

Downward movement of the long-handled tool (not shown), being connectedat its lower end 126 to the base 84, moves the fixture 82 from itsstarting position in FIG. 18 to its maximum lowered position in FIG. 19in which the four socket positioning elements 102 are seated on fourundamaged guide thimble upper end portions 44d. As the fixture 82 islowered, the compresser socket tool 94 and expander pin tool 96 movetoward the respective damaged upper end portions 44a,44b of the guidethimble 14a,14b into contact therewith. Engagement of the tapered loweredge 114 of the tool 94 and the tapered lower end 122 of the tool 96with the respective bent segments 80a,80b of the guide thimbles 14a,14binitiates corrective bending of the segments back to their originalpositions concurrently as the fixture 82 and elements 102 tools 94,96therewith are lowered. Each of the damaged guide thimble upper endportions 44a,44b are forceably returned and plastically set to aneffective outside diameter that permits subsequent entry into thepassageways 42 of the top nozzle adapter plate 40. In such manner, thetools 94,96 being adapted to rectify the two types of repairabale damageare operated to rectify the corresponding types of guide thimble segmentdamage.

In addition, the above-described movement of the fixture 82 toward theguide thimbles 14 places the cutter tool 98 within the irreparablydamaged upper end portion 44c of the guide thimble 14c with its cutterblades 130 at an operative position to sever the section 124 of theguide thimble which contains the irreparably damaged segment 80c. Thecutter tool 98 is connected with and operated by the lower end 132 of along-handled cutter operating tool (not shown). When the damaged section124 has been severed, the radial advancement of the cutter blades 130captures the cut-off section for removal when the fixture 82 iswithdrawn from the work station 68, as shown in FIG. 20.

TOP NOZZLE/GUIDE THIMBLE NON-STRUCTURAL JOINTS

Whereas the first and second types of damage have now been corrected andthe repaired upper end portions 44a,44b of the guide thimbles 14a,14bare ready to receive the top nozzle adapter plate 40 to completestructural joints therewith, the severed upper end portion 44c of theguide thimble 14c cannot complete such a joint with the top nozzle.Instead without some additional corrective measure, a open space or gapwill be left between the severed upper edge 134 of the guide thimble 14cand the adapter plate 40, allowing disruptive coolant cross flow throughthe gap and across the open upper end of the severed guide thimble 14c.

Turning now to FIGS. 21 through 24, the solution provided by the presentinvention is to mount an adapter sleeve 136 in each of the passagewaysin the top nozzle adapter plate 40 that is aligned with a severed guidethimble 14c. The adapter sleeve 136 is of a length longer than that ofthe damaged section 124 severed from the guide thimble 14c. An upperportion 138 of the adapter sleeve 136 has generally the same length andoutside diameter size as that of the severed section 124, whereas alower portion 140 is much shorter in length but has a larger outsidediameter. The lower portion 140 of the adapter sleeve 136 is sized tosnugly fit over the severed upper end portion 44c of the severed guidethimble 14c, as seen in FIG. 24, and provide a slip fit joint therewithin which there is no rigid connection between the sleeve 136 and guidethimble 14c when the top nozzle 22 is replaced back on the undamaged andrepaired upper end portions 44d,44a of the guide thimbles 14. By way ofexample, the adapter sleeve 136 can be approximately 1.5 inches longwith the same inside and outside diameters as the severed section 124over its upper portion 138 which is approximately one inch in length.The 0.5 inch lower portion 140 is expanded to a diameter that allows aclose-clearance fit with the outside diameter of the remaining upper endportion 44c (the portion of the insert sleeve which remains fastened tothe guide thimble 14c).

The above solution requires no changes to the manufactured top nozzle,but only the mechanical attachment of the one (or more) adapter sleeve136 to the adapter plate 40. The sleeve is permanently connected to theadapter plate by a simple roll-formed circumferential bulge 142 formedin the sleeve 136 so as to extend into the adapter plate passagewaygroove 48. The nature of the modification permits using the original topnozzle 22 removed from the fuel assembly 10 or a new replacement nozzle,if desired.

The adapter sleeve 136 does not provide a structural joint between thetop nozzle 22 and respective one guide thimble 14c of the fuel assemblyskeleton. However, because of conservative load margins for structuraljoints connecting the top nozzle to the skeleton, some of the structuraljoints (provided by the attaching structure 46 in FIG. 7) can beeliminated without violating design criteria. Since the adapter sleeve136 forms part of a non-structural joint, generally designated 144 inFIG. 24, it carries no load. The load of the fuel assembly 10 is carriedby the remaining structural joints (attaching structure) 46. Thesestructural joints 46 each support a greater load than they did beforethe damage occured when the full complement of joints shared the load.However, depending upon the fuel configuration, at least four to sixsections 124 can be deleted without exceeding design limits on theremaining structural joints 46. To state it another way, in a fuelassembly having 24 guide thimbles, there should be at least three timesas many structural as non-structural joints between the top nozzle andthe guide thimbles.

The adapter sleeve 136 is made of the same material, stainless steel, asthe section 124 of the upper end portion 44c (or insert sleeve). Theadapter sleeve 136 requires no locking tube 50, although locking tubesare inserted into the sleeves to maintain the same inside dimensions asnormal top nozzle joints.

The adapter sleeve 136 can be quickly installed, remotely, under waterinto irradiated nozzle adapter plates 40 or they can be installed intonew, unirradiated replacement nozzles by a "hands-on" procedure. Afterthe number of adapter sleeves 136 required have been mounted in thecorresponding adapter plate passageway 42 which are aligned with thesevered guide thimble 14c, the top nozzle 22 can be mounted to theskeleton in the normal manner using standard reconstitution fixturing,tooling, and in accordance with the same procedures developed forstandard removable top nozzle handling. The aforementioned modificationin no way compromises subsequent reconstitutions which may be necessaryfor that fuel assembly.

Therefore, the purpose of the adapter sleeve 136 is to bridge the gapbetween the top of the severed upper edge 134 of the guide thimble 14cand the adapter plate 40, and thereby provide a continuous protectiveenclosure for the core component installed in the fuel assembly, i.e.,either a control rod, a thimble plug, or a BPRA rod. The adapter sleeve136 precludes cross flow against the core component and maintains thenormal upward flow path of the primary coolant both inside and outsideof the guide thimble. In all probability, the fuel assembly skeletonwill be provided with a large majority of structural joints 46 and asmall minority of non-structural joints 144 between its top nozzleadapter plate 40 and its guide thimbles 14.

It is thought that the present invention and many of its attendantadvantages will be understood from the foregoing description and it willbe apparent that various changes may be made in the form, constructionand arrangement thereof without departing from the spirit and scope ofthe invention or sacrificing all of its material advantages, the formhereinbefore described being merely a preferred or exemplary embodimentthereof.

We claim:
 1. In a fuel assembly including a top nozzle, a plurality oflongitudinally-extending guide thimbles having upper ends and meansinterconnecting said top nozzle and said upper ends of said guidethimbles, said interconnecting means comprising:(a) a plurality ofstructural joints each being composed of means rigidly interconnecting aselected one of said guide thimble upper ends to said top nozzle; and(b) at least one non-structural joint being composed of a hollow sleeverigidly connected to said top nozzle and extending between said topnozzle and said upper end of one of said guide thimble being spacedbelow said top nozzle, said sleeve being disposed in a slip fitrelationship to said guide thimble upper end, said sleeve having anupper portion of outside and inside diameter sizes substantially thesame as that of said guide thimble upper end and a lower portion of aninside diameter size slightly larger than the outside diameter size ofsaid guide thimble upper end for receiving said guide thimble upper endtherein in a close fitting relationship.
 2. The fuel assembly as recitedin claim 1, wherein said upper portion of said sleeve is rigidly joinedto said top nozzle and said lower portion of said sleeve slidablyreceives said guide thimble upper end portion therein in said closefitting relationship.
 3. The fuel assembly as recited in claim 2,wherein said sleeve is bulge fitted in a passageway defined in said topnozzle.
 4. In a fuel assembly, the combination comprising:(a) a topnozzle; (b) a plurality of longitudinally-extending guide thimbleshaving upper ends; (c) a plurality of fuel rods; (d) a plurality ofsupport grids axially spaced along and supported by said guide thimblesand in turn supporting said fuel rods in a spaced apart array, anuppermost one of said grids being spaced below said top nozzle; and (e)a plurality of joints interconnecting said top nozzle and said upperends of said guide thimbles, all but at least one of said joints beingstructural joints providing rigid connections between all but at leastone of said guide thimble upper ends and said top nozzle, said one jointbeing a non-structural joint; (f) said non-structural joint beingcomposed of a member rigidly connected to said top nozzle and beingdisposed in a spaced nonattached relationship to, and above, saiduppermost grid and in a slip fit relationship to said upper end of oneof said guide thimbles being spaced below said top nozzle and above saiduppermost grid.
 5. The fuel assembly as recited in claim 4 wherein thenumber of structural joints constitutes at least a substantial majorityof said joints.
 6. The fuel assembly as recited in claim 4, wherein thenumber of structural joints is at least three times the number of saidnon-structural joints.
 7. The fuel assembly as recited in claim 4,wherein said member of said non-structural joint is a sleeve having anupper portion joined to said top nozzle and of a diameter sizesubstantially the same as that of said guide thimble upper end and alower portion of a diameter size slightly larger than that of said guidethimble upper end for slidably receiving said guide thimble upper endtherein in a close fitting relationship and being disposed in spacedrelationship to, and above, said uppermost grid.
 8. The fuel assembly asrecited in claim 7, wherein said sleeve is bulge fitted in a passagewaydefined in said top nozzle.